Ntp how many servers

Server administrators are advised to be prepared for this and take measures outside of the NTP protocol to drop packets from misbehaving clients when these clients are detected. Kiss-o'-Death KoD packets can be used in denial of service attacks.

Thus, the observation of even just one RATE packet with a high poll value could be sign that the client is under attack. And KoD packets are commonly accepted even when not cryptographically authenticated, which increases the risk of denial of service attacks. The broadcast server and all its broadcast clients share a symmetric cryptographic key, and the broadcast server uses this key to append a message authentication code MAC to the broadcast packets it sends.

Importantly, all broadcast clients that listen to this server have to know the cryptographic key. This mean that any client can use this key to send valid broadcast messages that look like they come from the broadcast server. Thus, a rogue broadcast client can use its knowledge of this key to attack the other broadcast clients. For this reason, an NTP broadcast server and all its clients have to trust each other. In symmetric mode, two peers Alice and Bob can both push and pull synchronization to and from each other using either ephemeral symmetric passive mode 2 or persistent symmetric active NTP mode 1 packets.

The persistent association is preconfigured and initiated at the active peer but not preconfigured at the passive peer Bob. Upon receipt of a mode 1 NTP packet from Alice, Bob mobilizes a new ephemeral association if he does not have one already.

This is a security risk for Bob because an arbitrary attacker can attempt to change Bob's time by asking Bob to become its symmetric passive peer.

As computing becomes more ubiquitous, there will be many small embedded devices that require accurate time. These devices may not have a persistent battery-backed clock, so using NTP to set the correct time on power-up may be critical for proper operation.

These devices may not have a traditional user interface, but if they connect to the Internet they will be subject to the same security threats as traditional deployments. Vendors of embedded devices are advised to pay attention to the current state of protocol security issues and bugs in their chosen implementation, because their customers don't have the ability to update their NTP implementation on their own.

Those devices may have a single firmware upgrade, provided by the manufacturer, that updates all capabilities at once. This means that the vendor assumes the responsibility of making sure their devices have an up-to-date and secure NTP implementation. Vendors of embedded devices with preconfigured NTP servers need to carefully consider which servers to use.

There are several public-facing NTP servers available, but they may not be prepared to service requests from thousands of new devices on the Internet. Vendors are encouraged to invest resources into providing their own time servers for their devices to connect to.

Vendors should read [RFC] , which advises against embedding globally-routable IP addresses in products, and offers several better alternatives. Anycast is described in BCP Also see [RFC]. With anycast, a single IP address is assigned to multiple servers, and routers direct packets to the closest active server.

Anycast can also be used in large organizations to simplify configuration of many NTP clients. Each client can be configured with the same NTP server IP address, and a pool of anycast servers can be deployed to service those requests. New servers can be added to or taken from the pool, and other than a temporary loss of service while a server is taken down, these additions can be transparent to the clients.

Note well that using a single anycast address for NTP presents its own potential issues. It means each client will likely use a single time server source. A key element of a robust NTP deployment is each client using multiple sources of time.

With multiple time sources, a client will analyze the various time sources, selecting good ones, and disregarding poor ones. If a single Anycast address is used, this analysis will not happen. This can be mitigated by creating multiple, separate anycast pools so clients can have multiple sources of time while still gaining the configuration benefits of the anycast pools. If clients are connected to an NTP server via anycast, the client does not know which particular server they are connected to.

As anycast servers enter and leave the network, or the network topology changes, the server a particular client is connected to may change. This may cause a small shift in time from the perspective of the client when the server it is connected to changes. In extreme cases where the network topology is changing rapidly, this could cause the server seen by a client to rapidly change as well, which can lead to larger time inaccuracies.

Configuration of an anycast interface is independent of NTP. Clients will always connect to the closest server, even if that server is having NTP issues. If the server is not performing to specification, it should remove itself from the Anycast network. Stratum 1 NTP servers can be deployed with unicast interfaces at several sites. Each site may have several Stratum 2 servers with two ethernet interfaces, or a single interface which can support multiple addresses.

One interface has a unique unicast IP address. The second has an anycast IP interface with a shared IP address per location. The unicast interfaces can be used to obtain time from the Stratum 1 servers globally and perhaps peer with the other Stratum 2 servers at their site. Clients at each site can be configured to use the shared anycast address for their site, simplifying their configuration. Keeping the anycast routing restricted on a per-site basis will minimize the disruption at the client if its closest anycast server changes.

Each Stratum 2 server can be uniquely identified on their unicast interface, to make monitoring easier. Time is a fundamental component of security on the internet. The absence of a reliable source of current time subverts many common web authentication schemes, e. Much of this document directly addresses how to secure NTP servers. In particular, see Section 2 , Section 4 , and Section 5. There are several general threats to time synchronization protocols which are discussed in [RFC].

Readers are encouraged to check the status of the draft, and make use of the methods it describes. This appendix contains additional recommendations specific to this implementation that are valid at the time of this writing. In addition to the recommendation given in Section 3. Starting with ntp The 'minclock' and 'maxclock' options of the 'tos' command may be used to override the default values of how many servers are discovered through the 'pool' directive.

In addition to NTP Control Messages the ntpd implementation also offers the Mode 7 commands for monitoring and configuration. If Mode 7 has been explicitly enabled to be used for more than basic monitoring it should be limited to authenticated sessions that provide a 'requestkey'. You can use the pool with any program speaking NTP, but if you are going to join the pool we recommend you use ntpd.

Beware that this might cause the number of queries to go up. Increasing the "time to live" on those records is recommended if the number of DNS queries is a concern, and will generally make the number of queries negligible. How do I join pool. Information for vendors The mailing lists Additional links. Caution: this option defeats the algorithms designed to cast out falsetickers and can allow these sources to set the sys- tem clock.

This option is valid only with the server and peer com- mands. From man chrony. It can be rejected as a falseticker in the source selection only if another source with this option does not agree with it. Log in to comment. Links seem to be broken does Redhat have an alternative source for those documents. Red Hat Active Contributor points. Paul Wayper. Hope this helps, Paul. US Community Member 87 points. UTS Support. SH Newbie 16 points. Shahid Habib. Hi, I have the similar problem in chrony, is there any parameter like "true"as it's ntp?

In , there were over seven million abusable NTP servers. As a result of software upgrades, repaired configuration files, or the simple fact that ISPs and IXPs have decided to block NTP traffic, the number of abusable servers dropped by almost 99 percent in a matter months, according to a January article in ACM Queue.

But there is still work to be done. In this blog post, I explore the challenges of NTP and prescribe some best practices for securing accurate time with this protocol. Computer scientist David L. Mills created NTP in the early s to synchronize computer clocks to a standard time reference. Since the inception of NTP, a group of volunteers with the NTP pool project have maintained a large, publicly available "virtual cluster of timeservers providing reliable easy to use NTP service for millions of clients" around the world for many Linux distributions and network appliances.

As detailed at NTP. For example, Stratum 0 serves as a reference clock and is the most accurate and highest precision time server e. Stratum 1 servers take their time from Stratum 0 servers and so on up to Stratum 15; Stratum 16 clocks are not synchronized to any source. The time on a client is established through an exchange of packets with one or more stratum servers.

These packets put timestamps on each message and the time taken to transmit the messages are factors in the algorithm for establishing the consensus for time that should be on the client. NTP can provide an accurate time source through consensus with multiple input servers. It can also identify which available time servers are inaccurate. One challenge is that NTP was built during a time when the Internet community was friendlier.

In the early s, it was common for NTP servers to be publicly available as a resource developers could use to troubleshoot and confirm their own NTP solution. This design decision created opportunities for abuse.

UDP, which is connectionless, is a best-effort protocol and, therefore, more susceptible to spoofing and losing packets than Transmission Control Protocol TCP. While NTP pool project volunteers have continued to advance NTP, the motivation for many network administrators and business owners to patch their own gear is not as strong. Since its inception, the NTP networking protocol has been integrated into countless systems, many of which haven't been patched and could be running code that is 20 revisions old or more.

NTP is how virtually every computer you interact with keeps its clock accurate, which is a function so fundamental to the functioning of the Internet that it can't be overstated What's more, vulnerabilities in NTP had turned the Internet's many time-servers into force-multipliers for Denial of Service attacks, making merely punishing attacks into nearly unstoppable ones.